The present invention relates to semiconductor photonic, discrete optic, integrated optic, and opto-electronic devices. In particular, the present invention relates to integrated optical modulators capable of modulation of the light beam intensity or phase by an electrical signal. Such modulators are required for converting electrical signals into optical signals so that the light beam can be used to transmit information over an optical communication system. The light source in an optical communication system is typically a semiconductor laser and the transmission of light is typically via an optical fiber.
The typical optical modulators available currently such as modulators based on Lithium Niobate (LiNbO3) crystals, free-carriers in silicon, or semiconductor quantum wells in compound semiconductors typically have high modulation voltage of around 5 Volts with around 6 dB (75%) device insertion loss (meaning only 25% of optical power will go through the device) with modulation frequency capable of going up to 40 Giga-Hertz (40 GHz) in which 1 GHz is 109 Hz. The radio-frequency (RF) power required to power up an optical modulator with a modulation voltage of 5V is P=V2/R in which R is typically the transmission-line impedance of 50 Ohms. Thus a 5V modulation voltage will correspond to a modulator RF power of 0.5 Watt (52/50 Watt). This power is very high especially when the modulator is used in an electronic-photonic integrated circuit (EPIC) or photonic integrated circuit (PIC), as the total power consumption of a large electronic microprocessor chip with millions of transistor is only a few Watts. Viewing the modulator as just one active device, its power requirement is extremely high when compared to a single electronic transistor, considering the fact that the typical power consumption of an electronic transistor in a typical micro-processor-type electronic chip is in the 0.2 to 1 micro-Watt (200 to 1,000 nanoWatts) range per transistor at the operating speed of 1 GHz. Also CMOS voltage is rapidly going below 1 Volt. Thus, modulator capable of operating at below 1 Volt would be of great interest to make it compatible with CMOS circuits. While there have been efforts on reducing the modulation voltage, they are typically achieved by making the device length longer, which often leads to high optical loss (>75% loss). Beside the undesirable long device length, high loss and “low voltage” is not very useful as it would mean a higher power laser would be needed to gain back the same optical power modulation and higher power laser will consume high electrical power as well.
While there are various modulator devices with different functionalities, they can share the same general device structure that give high device operating efficiency or device performance (e.g. modulator capable of ultra-low voltage, broad modulation bandwidth, low optical loss). This general device structure capable of giving high device efficiency or high device performance is the focus of the present invention.
Another area of applications is called Radio-Frequency Photonics (RF Photonics). RF Photonics enables high frequency electrical signal to be transported via modulating an optical beam and transmit it through an optical fiber. The electrical signal is then recovered with a high-speed optical power detector. In RF photonics, it is also desirable to have modulator voltage going below 1 Volt (preferably below 0.5 Volt) with a low device throughput loss of <75% for the optical beam. In all these applications, high modulation bandwidth of exceeding 1 Giga-Hertz's (GHz) up to tens of GHz preferably reaching 100 GHz is generally desirable as well. In many on-chip applications, it is desirable that the modulator's physical length be shorter than around 1 mm. While the prior arts on optical intensity or phase modulators are able to either reach one or two of the desirable properties such as high modulation bandwidth (>1 GHz) or low optical-power throughput loss (<75%), they are typically not able to reach all the desirable properties that include low modulation voltage, high modulation bandwidth, low optical throughput loss, and compact device size all in one optical intensity or phase modulator device.
Modulator Figure of Merits and the Advantages of the Present Invention
As noted, achieving low modulator voltage VMOD alone is not sufficient for these applications. The modulator must have low device insertion loss (i.e. high optical throughput power) or high optical power transmissivity T defined as the modulator's output optical power over input optical power TMOD=(optical power output)/(optical power input). Any loss in the modulator device's optical throughput power is equivalent to low electrical signal power to optical signal power conversion, which is an important factor to be considered in the figure of merit for the desirable modulators. As the amount of optical power modulated is proportional to the modulator voltage, in terms of the signal to noise transmitted, 1/TMOD2 is equivalent to VMOD2 (or 1/TMOD is equivalent to VMOD), which is also an important factor to be considered in the figure of merit for the desirable modulators. That is, a reduction in TMOD by a factor of 2 is equivalent to an increase in VMOD by a factor of 2, and is equally undesirable for achieving low signal to noise ratio in the signal modulation and transmission. Similar arguments go to the modulation depth (MD) of the modulators defined as the percentage of the optical power being changed by the modulator. In terms of the signal to noise transmitted, 1/(MD2) is equivalent to VMOD2, and shall be an important factor to be considered in the figure of merit for the desirable modulators.
Typically, the higher the modulator bandwidth BW (in GHz), the better the modulator, and is a factor to be considered in the figure of merit. Also, the shorter the modulator device length LMOD, the better it is. There is typically a linear tradeoff between device length and modulator voltage so (LMOD2) is equivalent to VMOD2 as a factor to be considered in the figure of merit. Finally, the maximum optical power MP (in milliWatt) that can be taken in by the modulator without losing modulation performance is also important, and is a factor to be considered in the figure of merit. This is some time called the modulator's optical saturation power and the modulation saturation can occur due to carrier excitation by the high optical power passing through the modulator material. In term of electrical signal to optical signal power transfer, the higher the optical power, the more the electrical signal can be converted back to RF voltage from the modulated optical beam at the photodetector end. Hence 1/(MP2) is equivalent to VMOD2 as a factor to be considered in the modulator figure of merit. Bandwidth (BW) and modulation power (or energy per unit time) are inversely proportional in figure of merit. Note that 2× the modulation bandwidth means half RF energy per data transmitted as the time period is halved. Thus signal to noise is actually worse by 2× though data rate is 2× faster—meaning the advantage of higher modulation is sort of cancelled off by the higher noise but we will not exactly account for the noise in the figure of merit. All faster pulses suffer the same low power and high noise trade off (i.e. it is not intrinsic to the modulator). We will still see larger BW as an advantage like smaller RF power (1/VMOD2).
Combining all the above-mentioned factors, when comparing the modulator performances, it is useful to compare them in terms of the following “Modulator Figure of Merits” defined in terms of the quantities mentioned above as follows:MFOM=TMOD2×MD2×MP2×BW/(VMOD2×LMOD2),  (1)
where “x” in the above equation means mathematical “multiplication” and “/” means mathematical “division”. For the discussion in this patent, we will ignore the MP factor for simplicity (i.e. we will set MP=1). The squaring in VMOD is because of power consideration. The other factors are done in its equivalent to VMOD. Defined this way, the higher the value of MFOM, the better the modulator for the purpose of achieving large frequency bandwidth, low device's optical power throughput loss, small device size, low modulation voltage, high modulation depth, and high device's optical power tolerance. This MFOM shall be compared with that of an “ideal optical intensity and phase modulator” defined below. For the purpose of applying to RF Photonics and EPICs, we define a reference “ideal optical modulator” to be one that has the following characteristics: TMOD=0.25, VMOD=0.5 (V), LMOD=1 (mm), BW=40 (GHz), MD=0.9. The MFOM of such an ideal modulator denoted as “MIFOM” is then given by:MIFOM=0.252×0.92×402/(0.52*12)(mW2×GHz/(V2×mm2))=8(GHz/(V2×mm2)).  (2)
The ratio between the MFOM of an optical modulator and the MIFOM is of particular interest as it would be an indication of how close the modulator in question is to an “ideal modulator” or how much it surpasses the ideal modulator. This ratio may be referred to as the Modulator's Relative to Ideal FOM or relative modulator figure of merit (RMFOM) defined as follows:RMFOM=(MFOM/MIFOM).  (3)
When this value is close to 1 or higher than 1, the optical modulator is said to compare favorably with the target ideal modulator in terms of the modulator performances. On the other hand, when this value is much smaller than 1, the optical modulator is not good compared with the performances of the target ideal modulator.
A typical performance for a “compound semiconductor based” modulator of the prior arts has TMOD=0.25, VMOD=5 (V), LMOD=2 (mm), BW=40 (GHz), MD=0.9. The MFOM of such a typical modulator of the prior art is given by:MFOM=0.252×0.92×40/(52*22)(GHz/(V2×mm2))=0.02(GHz/(V2×mm2)).  (4)Its RMFOM is then given by:RMFOM=0.02/8=0.0025=(1/400).  (5)
This is 400 times worse than the targeted ideal optical modulator. The prior arts for realizing optical modulators are thus highly inadequate in terms of realizing the targeted performances of the ideal modulator.
A typical performance for a “silicon based” modulator of the prior arts has TMOD=0.25, VMOD=2 (V), LMOD=5 (mm), BW=10 (GHz), MD=0.9. The MFOM of such a typical silicon modulator of the prior arts is then given by:MFOM=0.252×0.92×10/(22*52)(mW2×GHz/(V2×mm2))=0.005(mW2×GHz/(V2×mm2)).   (6)Its RMFOM is then given by:RMFOM=0.005/8=0.000625=(1/1600).  (7)This is 1600 times worse than the targeted ideal optical modulator. The prior arts for realizing silicon based optical modulators are thus highly inadequate in terms of realizing the targeted performances of the ideal modulator.
In the present invention, the major limitations of the prior arts are overcome, making it possible for the optical modulator of the present invention to have RMFOM ranging from 0.01 to over 2.5, which is generally over 10 to 1,000 times higher in term of figure of merit than the typical optical modulators based on the prior arts. This is a very significant advantage for the present invention over the optical intensity or phase modulators based on the prior arts.
Exemplary Modulators in the Prior Art
An exemplary embodiment of the present invention for the modulation material is based on utilizing the near bandgap effect of refractive index change due to carrier band-filling and other electro-optic effects in compound semiconductor, resulting in electro-optic (EO) modulators. Another exemplary embodiment of the present invention for the modulation material is based on utilizing the electro-absorption effects in compound semiconductor, resulting in electro-absorption (EA) modulators.
Electro-absorption modulation of an optical beam is based on optical absorption change due to an applied voltage, which as is well known to those skilled in the art. Electro-absorption modulation can be induced by a shift in the semiconductor bandgap energy under an applied voltage or induced by carrier injection-depletion that can change the optical absorption or gain property of the semiconductor materials. These mechanisms for electro-absorption modulation are relatively straight-forward and well known in the prior art.
For electro-optic modulator, the mechanisms involved are more complex. It is thus of interest to review the physics of refractive index change in silicon-based modulators and compound semiconductor based modulators below.
Physics of Refractive Index Change in Silicon Based Modulators
The main difference between compound semiconductor based modulators and silicon modulators for operations at the 1550 nm wavelength range for fiber-optic communications is that while silicon has an absorption band gap, it is at 900 nm, which is far away from the operating wavelength of 1550 nm. Hence, the refractive index change in silicon is not due to effects related to the band-gap energy but due to free-carrier plasma effect. Free-carrier plasma can change the refractive index over a broad wavelength range but also will cause optical absorption. Hence, free-carrier plasma effect has a drawback in that when the refractive index change due to the free carriers is high, the amount of free-carrier absorption is also high so there is a significant amount of optical absorption that reduces the optical beam intensity when the phase shift induced by the free-carrier plasma is high. In silicon, the refractive index change due to the free-carrier plasma effect is known to those skilled in the art as given approximately by:ΔnPlasma=−(8.8×10−22ΔNe+8.85×10−18ΔNh0.8),  (8)where ΔNe is electron carrier density in (1/cm3), and ΔNh is hole carrier density in (1/cm3). For example, with a carrier density of 1018/cm3, the refractive index change of the active P-N junction region denoted by ΔnPlasma can be around ΔnPlasma=0.002 for holes and ΔnPlasma=0.0009 for electrons. These are relatively low values that limit the low-voltage performances of silicon-based optical modulators.Physics of Refractive Index Change in Compound Semiconductor Based Modulator
Below, a material is said to be an active modulator material or more precisely an active electro-optic (EO) material if the material's refractive index can be altered by an applied voltage, an electric current, or either injection or depletion of carriers. A material is said to be an active electro-absorption (EA) material if the material's optical absorption or optical gain can be altered by an applied voltage, an electric current, or either injection or depletion of carriers. The active EO or EA materials are more generally referred to as active modulator material or medium (ACM), or as active area, or simply as active medium. In an active EO material, the refractive index change can then be used to shift the optical phase of an optical beam or change the intensity of the optical beam (using for example a Mach Zehnder interferometer as is well known to those skilled in the art). The modulation is said to result from electro-optic modulation (as oppose to electro-absorption modulation for which the optical absorption of the modulator material is altered). An advantage of EO modulator in general is that it can have intensity change that is somewhat linear as a function of the applied voltage, or can be corrected by an external electrical circuit to make the modulation linear. Such linear intensity modulation capability is important for applications to Radio-Frequency Photonics (RF Photonics) area.
In compound semiconductor, the energy bandgap of the compound semiconductor, whether it is bulk compound semiconductor or quantum wells, can be designed to be close to the wavelength of operation. For example, in some situation, such as electro-optic modulation, it is advantages to design the bandgap to be at 1350 nm for operation at 1550 nm wavelength range. In other situation, such as electro-absorption modulation, it is advantages to design the bandgap to be at 1450 nm for operation at 1550 nm wavelength range. The prominent refractive index change effect most frequently utilized by the prior art is quantum confined stark effect (QCSE), which changes the refractive index due to strong electric field applied across the quantum wells that shifts the transition energy of the quantum wells, which then leads to a change in the refractive index. It is purely an electric field effect and does not involve electron or hole carriers. The refractive index change due to QCSE is given as:ΔnQCSE=(½)n3KQCSEE2,  (9)
where KQCSE=˜0.3×10−18 m2/V2 for InGaAsP quantum wells, n is the refractive index of the quantum well material, and E is the applied electric field in Volt/meter. QCSE is a quadratic electro-optic (QEO) effect as it depends on the electric field square. Another refractive index shift effect is due to the intrinsic electro-optic coefficient of the compound semiconductor material called Pockels effect. This effect is also referred to as linear electro-optic effect (LEO) of the crystal. For InP, it is given by:ΔnPockels=(½)n3r41E,  (10)
where r41=˜2×10−12 m/V for InP material. Assuming a 2V applied voltage across a 200 nm thick active modulator material giving an electric-field strength E=1V/(100 nm)=107 V/m, then the above with n=3.4 would give ΔnQCSE=0.0005 and ΔnPockels=0.00033. At a higher voltage of 4 V, QCSE would overtake Pockels effect substantially and becomes the dominant effect giving ΔnQCSE=0.002. Thus in the typical compound semiconductor modulator design, QCSE is the main effect used for achieving refractive index shift. Both QCSE and Pockels effect just need an applied field (i.e. no carrier is needed), which is often seen as an advantage to achieve low optical loss (carrier can cause free-carrier optical absorption loss).
Other effects that are seldom used in compound semiconductor based modulators are dependent on electron or hole carriers. One effect is the carrier plasma effect, which is also used by silicon modulators as discussed above. The refractive index change due to the carrier plasma (PL) effect is given by:ΔnPlasma=[(−e2λ02)/(8π2c2∈0n)]*(ΔNd/me*).  (11)It has a value similar to Pockels effect under the same applied field if the quantum well (or bulk material) doping Nd is n-doped with an electron doping density of around Nd=Ne=1×1017/cm3. Another effect is due to carrier filling up the conduction bands (or valence bands). It is called the band filling (BF) effect. FIG. 1a illustrates the case for which the electron carriers fill up the conduction band, leading to a shift in the absorption energy from close to the bandgap energy Eg to larger than the bandgap energy Eg+ΔE (or in wavelength Δg−Δλ). As shown in FIG. 1b, this change in the “absorption energy edge” from absorption curve αEg(λ) to αEg+ΔE(λ) leads to a change in the refractive index of the material Δn(λ) due to what is known to those skilled in the art as the Kramer's Krognig's relation which says that a change in the absorption spectrum Δα(λ)=αEg+ΔE(λ)−αEg(λ) as a function of the wavelength (λ) must lead to a change in the spectrum for the refractive index Δn(λ) as a function of the wavelength. This results in a change in the refractive index at the operating wavelength of the beam at λB (say at 1550 nm) or its corresponding beam photon energy EB due to the absorption edge change (say at λg=1350 nm) because of carriers filling up the semiconductor energy bands (e.g. it can be electrons filling the conduction bands or holes filling the valence bands). The refractive index change at the operating wavelength can be expressed as:ΔnBandFil=RBandFil(λ)×ΔNe,  (12)where the proportionality coefficient RBandFil(λ) is dependent on the optical wavelength λ and its value would be high at close to the bandedge compared to its value at away from the bandedge. At a doping density of Ne=1017/cm3, ΔnBandFil can be 2-3 times higher than ΔnPlasma or ΔnPockels under the same applied voltage for the III-V (e.g. InP/InGaAsP) material system when the bandedge is at 1350 nm or 200 nm away from the operating wavelength of 1550 nm but may be lower than nQCSE at a high enough applied voltage V as QCSE scales as the square of the electric field and is proportional to V2, whereas other changes in the refractive index including ΔnBandFil are only proportional to V. Normally ΔnBandFil cannot be too high in the optical modulator structures of the prior arts due to a few reasons.
First, ΔnQCSE can always be dominating at high enough voltage or high enough applied field as it is proportional to V2.
Second, in the prior art, the quantum well (or bulk) active material (ACM) is either undoped or lowly doped with carriers having a doping density typically lower than about 1×1017/cm3. The no doping or low doping is due to the problem in the modulator designs of the prior art that high carrier doping density will substantially increase the optical absorption loss of a light beam going through the entire modulator. In the modulator design of the prior art, the optical loss would occur not only just at the active modulator section at which the refractive index or absorption is designed to change under an applied voltage but also in the rest of the connecting region such as in a Mach-Zehnder interferometer geometry. This results in a long waveguide section all doped with the high carrier doping density, which then results in high optical loss due to the high free-carrier density from the doping. For example, while the active modulator region may have a length of 0.5 mm, the entire modulator with the Mach-Zehnder interferometer geometry may have a total physical beam propagation length of 2-5 mm.
Third, a highly doped quantum well region will decrease what is known to those skilled in the art as the carrier depletion width dPN of the P-N junction, a region between the P-doped region and N-doped region in which no or few carriers exist. The smaller the depletion width dPN, the larger the device capacitance as the width defines the dielectric width of a capacitance in which one capacitor plate is the N-doped region and another capacitor plate is the P-doped region, and these two plates are separated by the depletion width dPN. As is known to those skilled in the art, the smaller the “plate separation”, the larger the capacitance CPN. The large P-N junction capacitance CPN will drastically reduce the modulation bandwidth of the optical modulator partly due to RC frequency cutoff, where R will be the effective series resistance of the device Rser and the RC modulator frequency cutoff will be fRC=1/(2πRserCPN). Thus the quantum well in prior art is typically undoped and if it is doped, it is kept to a doping density of 1×1017/cm3 or lower or it would be hard to achieve large modulation bandwidth for the modulator due to higher device capacitance CMOD that would lead to low modulator RC cutoff frequency fRC, given the typical device series resistance RSer achievable in the prior art. The 1×1017/cm3 doping level gives approximately a depletion width around 100-300 nm under zero to a few volts of applied voltage, which is about the active region and waveguide core thickness of a conventional semiconductor based modulator. Going to higher doping than 1×1017/cm3 will be regarded as inefficient design as the depletion width will be too small to cover the active region and waveguide core thickness, and also will unnecessarily lower the modulation bandwidth due to the higher junction capacitance. In the present invention, this limitation is overcome, resulting in substantially lower modulation voltage while maintaining high modulation frequency cutoff.
An Exemplary Modulator in Prior Art
FIG. 2 shows an electro-optic (EO) modulator of prior art. In particular a modulator made of III-V compound semiconductor material. An electro-absorption modulator in the prior art will have a similar optical and electrical structure except that the active material is replaced by a material that can cause electro-absorption (i.e. a material whose optical absorption can be altered under an applied voltage, an electric current, or either injection or depletion of carriers). For the purpose of illustration of a modulator of prior art, we will describe an EO modulator.
The EO modulator utilizes semiconductor quantum well's quantum-confined stark effect as the main electro-optic (EO) effect. Under an applied electric field, the quantum well's width effectively narrows and pushes the energy level higher, resulting in a decrease in the refractive index due to increase in the energy bandgap. The direction of the electric field does not matter so it is a quadratic electro-optic effect. For 1550 nm operation, a typical modulator structure is shown in FIG. 2 showing device 10000. In device 10000, the device is fabricated on a semiconductor substrate SUB 10010. In an exemplary device substrate SUB 10010 is N-doped InP with N-type doping density of N=3×1018/cm3. Above the substrate is a lower electrical Ohmic contact layer LOHC 10020. In an exemplary device LOHC 10020 is N-doped InGaAs with N-type doping density of N=1×1018/cm3 and a thickness of 0.1 μm. Above the LOHC 10020 is a lower conducting waveguide cladding layer (LCWCd) 10030. In an exemplary device LCWCd 10030 is N-doped InGaAs with N-type doping density of N=1×1018/cm3 and a thickness of 1.5 μm. Above the LCWCd 10030 is a lower waveguide core separate confinement heterostructure (SCH) layer (LWCoSCH) 10040. In an exemplary device LWCoSCH 10040 is N-doped InGaAlAs with energy bandgap wavelength of 1.3 μm and N-type doping density of N=1×1017/cm3, a thickness of 0.1 μm. Above the LWCoSCH 10040 is an active electro-optic and waveguide core layer (AEOWCo) 10050. In an exemplary device AEOWCo 10050 is comprising of 14 quantum wells (8 nm thick) and 15 barrier layers (5 nm thick) made of InGaAlAs material with no or low doping (called intrinsic semiconductor or I-type semiconductor), resulting in a total thickness of 0.182 μm for layer AEOWCo 10050. The quantum well layer has 35% compressive strain with respect to InP lattice and the barrier layer has 40% tensile strain with respect to InP lattice. Above the AEOWCo 10050 is an upper waveguide separate confinement heterostructure (SCH) layer (UWCoSCH) 10060. In an exemplary device UWCoSCH 10060 is P-doped InGaAlAs with energy bandgap wavelength of 1.3 μm and P-type doping density of P=1×1017/cm3, a thickness of 0.1 μm. Above the UWCoSCH 10060 is an upper conducting waveguide cladding layer (UCWCd) 10070. In an exemplary device UCWCd 10070 is P-doped InP with energy bandgap wavelength of 0.9 μm and P-type doping density of P=1×1018/cm3, a thickness of 1.5 μm. Above the UCWCd 10070 is a upper electrical Ohmic contact layer UOHL 10080. In an exemplary device UOHC 10080 is P-doped InGaAs with P-type doping density of P=1×1019/cm3 and a thickness of 0.1 μm. Above the UOHC 10080 is an upper metal contact layer UMC 10090. In an exemplary device UMC 10090 is single or multi-layer metal denoted by layer UM1 10091, UM2 10092, UM3 10093, . . . with UM1 layer directly on top of UOHC 10080. In one exemplary embodiment, UM1 is 20 nm of Ti, UM2 is 50 nm of Pt, UM3 is 1000 nm of Au. On the lower side above the LOHC 10020 is a lower metal contact layer LMC 10100. LMC 10100 is single or multi-layer metal denoted by layer LM1 10101, LM2 10102, LM3 10103, . . . with LM1 layer directly on top of LOHC 10020. In one exemplary embodiment, LM1 is 17 nm of Au, LM2 is 17 nm of Ge, LM3 is 17 nm of Au, LM 4 is 17 nm of Ni, LM5 is 1000 nm of Au.
Summary of Limitations of Prior Arts in Intensity or Phase Modulators
Below, we summarize further the limitations of prior arts in modulators by using the example of a semiconductor based modulator. The typical semiconductor modulators have switching voltages of around 2-5V with a device length of a few millimeters. The optical mode is confined by weakly-guiding structure in the vertical direction with effective mode size of 0.5-1 μm and thick optical cladding of ˜1.5 μm to prevent the guided optical energy from reaching the top or bottom metal electrode with high metal optical absorption loss. The waveguide core usually has quantum wells (QWs) to enhance the refractive index modulation under an applied electric field. Based on such structure, compound semiconductor EO modulators with 40 GHz modulation bandwidth have been achieved with use of QWs and a PIN (P-doped, Intrinsic (i.e. undoped or being an Intrinsic semiconductor material), N-doped) type structure, with V□□ of 2-3V (voltage that gives a total relative phase shift of □□ in the modulator Mach Zehnder Interferometer (MZI)). FIG. 3 shows the general cross-section of such a PIN modulator structure, which is a general schematic of the more detailed exemplary device structure shown in FIG. 2. The modulator performance gives a modulator voltage of 3V, a modulation bandwidth of 30 GHz, and a modulator length of 1 mm (see [REF 1]). All bandwidth noted here is analog 3 dB optical power bandwidth (not in digital Gb/s that needs smaller analog bandwidth).
There are typically four contributions to electro-optic modulation for a semiconductor modulator: (1) Linear Electro-Optic effect (LEO); (2) Quadratic Electro-Optic effect (QEO) including quantum confined stark effect (QCSE) when quantum wells are used; (3) Band-Filling effect (BF); (4) Plasma Effect (PL). These are well described in the literature. Most modulators with QWs utilize LEO+QCSE as the main effects. Some may involve BF and PL as additional effects.
The voltage is usually high (2-5V) in prior arts partly because these conventional EO modulators used a weakly confined waveguide structure with thick cladding of typically 1.5 micrometer thickness to avoid metal optical absorption loss. This resulted in a vertical optical mode with width of approximately 0.5 to 1 micrometer at full-width-at-half-maximum (FWHM) power points but the full optical energy is extended to 2-3 micrometers in size vertically. If the entire optical mode region is filled with active EO materials such as quantum wells (utilizing the QCSE), then the electrode spacing will be about 2 to 3 micrometers (or 2,000 nm to 3,000 nm) in spacing, which is large. The use of PIN structure can reduce the electrode spacing to say to around 200-300 nm, but then the percentage of the 200-300 nm thick electro-optic medium overlapping with the 1,000 nm large optical beam mode energy and the RF field (called the mode-medium overlapping factor) will be small (down to ˜10%). The applied RF electric field is usually somewhat uniformly distributed in the active electro-optic medium. Thus, the advantage of 10 times higher electric field (due to electrode spacing reduced from 2,000 nm to 3,000 nm down to 200 nm to 300 nm) is cancelled by the proportionately reduced mode-medium overlapping factor (from near 100% down to ˜10%). As a result, the modulation voltage is not changed much by the use of the PIN structure.
Due to one or more of the abovementioned reasons as illustrated via an EO modulator in the prior art, conventional semiconductor EO or EA modulators have high modulation voltages of 2-5V depending on its device length and the nonlinear electro-optic effects used among the four main effects (LEO, QCSE, BF, or PL effect). Typically 2V to 3V Vπ can be achieved with 2-3 mm long device. If all four effects are used, it could be reduced to ˜1 mm long device with 2V to 3V Vπ. The modulator voltage cannot be much smaller and the length cannot be much shorter and still maintain the high modulation bandwidth of 10 GHz or higher.
In the present invention, the above limitations of the prior arts are overcome, resulting in broadband low voltage modulators with small device size having a relative figure of merits, RMFOM, that are generally over 10 to 1,000 times higher than modulators in the prior arts.